1. Electrostatic immunity
Ideally, our system is a hollow and airtight metal box. According to the electromagnetic field theory, it is impossible for any external static source to affect the circuit operation inside the box. Such a system, however, is moot. An actual system must include an external interface (that is, it cannot be airtight). A practical (metal cased) system should be able to discharge static electricity applied to it to ground quickly enough (before it builds up an electrostatic charge large enough to affect system operation). This brings the following requirements (only for metal casing devices):
1) Good ground terminal
2) Any exposed metal that may be affected by static electricity should have a good connection with the ground terminal
3) Any gaps in the device case should be small enough that static electricity cannot “travel” into the device through the metal gap
2. Overall view of the chassis
Figure1: Overall view of the chassis
This chassis has a top cover which has a large area of metal contact with the chassis. The upper cover covers the left, right, upper and rear sides of the chassis as a whole.
3. Power input and AC/DC
Figure2：power input and AC/DC
The ground wire for power input is directly connected to the chassis shell, but not to the AC/DC. That is to say, the large piece of metal of the chassis is used as a perfect electrostatic discharge circuit (it should also be good for tests such as surges). The blue device above the AC/DC power input socket should be a varistor.
4. AC/DC output
The AC/DC output is directly connected to the motherboard (PCB power input and working place).
5.The Connection between the Motherboard and the Chassis
Figure3：The connection between the motherboard and the chassis
It can be seen that there is a large area of exposed copper foil under the screws used to fix the motherboard. Disassemble the main board, the chassis shell below is like this:
Figure4：The chassis shell under the motherboard
Each mounting hole protrudes a larger area. Correspondingly, viewed from the bottom of the chassis, the mounting holes are recessed.
Each mounting PCB screw connects a ground on the PCB (work ground or connector ground, see below) to the case shell. The working ground on the PCB and the connector ground are connected through the chassis shell.
6.The connection between the mainboard connector and the chassis and mainboard
There are two situations.
6.1 The connector welded on the main board is in good lap with the case shell
figure5：The condition of a good lap
In the figure, the chassis shell has a bend at the connector hole, which is well connected with the connector reed. At this time, connect the connector shell directly to the working ground of the PCB.
6.2 The contact between the connector shell and the chassis is not good enough
At this time, the connector shell uses a separate ground plane on the PCB (multiple connector shells with poor contact with the chassis shell share the ground plane), and then on the one hand, it is connected with capacitance between the PCB and the working ground, and on the other hand, it is attached to the chassis shell through the screws described above and large areas of copper foil. The connector ground and the working ground on the PCB share the ground through the chassis. On the PCB, these two are not directly connected (there is a capacitance connection between the two). In this way, when electrostatic discharge is performed on such connectors, the static electricity will first be discharged through the chassis shell and will not go to the working ground of the PCB.
Figure6：Conditions of poor contact
7.Miscellanea
7.1 Tact switch
The ground pin of the tact switch (operated through the panel) is connected to the connector ground, so the tact switch will not work if it is not installed in the chassis.
7.2 Network interface protection
In addition to the commonly used TVS on the local side, the ground on the transformer side is connected to the connector ground (finally connected to the working ground through the chassis) through a varistor (resistance value is unknown).
The network indicator light is placed on the PCB and led out through a long light guide column, instead of using an RJ45 socket with LEDs to avoid static electricity from interfering with the system operation through the LED signal line.